Experimental Study on the Bond Behavior of Bundled Bars

When a reinforced concrete structure is very loaded, the reinforcement congestion can occurred. In this situation, the use of bundles in two, three or four instead of individual bars could reduce or even avoid the reinforcement congestion, allowing an easier compaction of concrete. However, there is still a lack in knowledge of the fundamental phenomena related to the bond behaviour of bars in bundles. The different codes of practice (i.e. ACI 318-14 and Eurocode 2) consider a bundle (group of bars) as a single bar of equivalent diameter. This assumption have been evaluated in the present paper by means of an experimental program on a set of pull-out tests loaded on bundles (two or three bars) and their equivalent diameter. Results indicate that the bundles have a significantly greater loading capacity than their equivalent bars. Consequently, the codes lead to conservative predictions of the bond strength of bar in bundles. The possible equivalence in terms of minimum perimeters between a bundles and a single bar have been also evaluated by a second experimental program carried out on another set of pull-out specimens. The results showed that this equivalence leads to bond strength predictions closer to the ones experimentally observed, even though it resulted unconservative in some cases and therefore should not be adopted in practice.